The present invention relates to a transmission which has its rotating direction switched forward or backward during its run so that it may be used with a control apparatus such as an industrial robot.
The control apparatus such as the industrial robot is run with its rotating direction not limited to one direction but switched forward or backward as required. On the other hand, the gears of a transmission to be used with such control apparatus never fail to be accompanied by the back-lash. Because of the presence of this back-lash, there has been pointed a problem that the rotations are delayed to the extend of the back-lash, each time their direction is switched forward or backward, to make the transmission inaccurate.
In order to eliminate that inaccuracy of the transmission due to the back-lash, according to the prior art, there has been proposed a mechanism or the like, in which dual gears are superposed and shifted in phase by the back-lash so that their surfaces may come into meshing engagement with no back-lash. Despite of this proposal, however, the back-lash contains such an error as is inevitably caused by the machining. It is, therefore, extremely difficult to improve an indexing accuracy so that the two gears are fixed on their shafts with a phase shift of the back-lash having such fine error. Moreover, the two-gear superposed mechanism has failed to avoid the fatal wedge action which is caused by the two-surface meshing engagement, which in turn raises a cause for making a smooth run difficult.